The invention is in the field of telephone and data switching systems and more particularly is a telephone data switching system which is adaptable and is capable of switching analog and several different types of digital signals.
Switching systems in general have been in use for many years, the most common being the type used by local telephone companies for switching analog signals. Basically, these systems include a matrix of switches which selectively connect incoming lines to outgoing lines. Such switches are adaptable for continuous analog signals which emanate from the standard home telephone set. In recent years switches have been designed to switch digital signals. The digital signals typically represent data or digitally converted analog voice signals. The digital switching systems typically operate on a time divided basis wherein multiplexed digital channels are in a first sequential order at the input side of the switch but are in a second "switched" sequential order at the output side of the switch. Timing units divert the output digital channels into respective output lines for transmission to subscribers.
The most conventional format of the digital signals is the PCM format wherein the instantaneous amplitude of an analog signal, e.g., voice, is converted into a group of bits, typically an 8-bit signal, which digitally represents the analog amplitude. Other types of planned telephone sets use CVSD digital signals. The latter designation stands for continuously variable slope delta modulated signals and is a known form of delta modulation. In addition to the various types of digital signals used or planned for calls, there is an additional variable which one must consider in designing a switching system. Signals may arrive at a switching system on either trunks or loops. As is commonly used in the art, and as those terms are used herein, a loop simply means a line (input and output) which is the connection between a telephone instrument and the switching system. A trunk is simply a line (input and output) which is a connection between switches. A digital trunk is multi-channel. For example, a trunk may carry CVSD/TDM, analog/TDM, or PCM/TDM. Also, as will be apparent, clusters of digital telephones may be multiplexed to form a loop group. An additional variable faced by switching systems is that the incoming signals on the digital trunks may be synchronized to the timing of the switching system or they may be asynchronous with respect to the timing of the switching system. COnsequently, in the case of asynchronous signals some means must be provided for insuring that the incoming digital signals are time corrected so that they can be properly operated upon by the switching system.
In general, the evolution in the art has been from analog to asynchronous digital communications to planned synchronous digital communications. A problem with existing switching systems is that they cannot handle the evolution. Thus, typically, when a change over is made for several connections to a switch a new switching system has to be installed.